1. Field of the Invention
The present invention relates to an infrared photodetector and method thereof, and more specifically to an infrared photodetector using doping superlattice structure with improved photo-sensitivity and method thereof.
2. Description of the Prior Art
In general, infrared semiconductor devices are used in many practical applications such as communications, security systems, and other modern electronic devices. In many of these applications, the sensitivity of the infrared photodetector is critical to its application.
Accordingly, there have been many advances in the use of semiconductor and transistor structures for detecting infrared radiation or light. Many of these advances have resulted in faster, more responsive detectors, as well as detectors capable of detecting multi-color infrared radiation.
Recently, multiple quantum well (MQW) photodetectors using heterojunction superlattice structure of compound semiconductors have been developed One such detector is disclosed in U.S. Pat. No. 5,272,356 issuing to Wen on Dec. 21, 1993, and entitled "MQW Photodetector for Normal Incident Radiation".
However, infrared photodetector with doping superlattice structure have not yet found widespread commercial use.
The doping superlattice structure-consists of ultrathin n- and p-type doped layer, possibly separated by intrinsic layers (n-i-p-i crystal). These doped superlattice exhibit novel electronic properties which result from a very efficient spatial separation between electron and hole subbands (indirect gap in real space) by the periodic space-charge potential. This separation implies recombination lifetimes which may be increased by many orders of magnitude over those of bulk material. Consequently, large deviations of the electron and hole concentration from thermal equilibrium are metastable under weak excitation conditions.
In addition, the space-charge-induced superlattice potential itself is strongly affected by a variation of the electron and hole concentration. The effective energy gap of a n-i-p-i crystal as well as the structure of the dimensional (2D) subband should thus no longer be fixed material parameters but tunable quantities.
The basic electron and optical properties of the doping superlattice structure (n-i-p-i crystal) are tunable as a consequence of the spatially indirect bandgap which can be varied with optical or electrical excitation. The absorption coefficient of a semiconductor at photon energies below the bandgap increases in the presence of an electric field.
In a n-i-p-i crystal, a large electric field is built in due to the ionized impurities. Hence, by varying the effective bandgap which varies the electric field, the absorption coefficient and the conductivity can be tuned. Using these properties, infrared photodetector is manufactured.
FIG. 1 shows a cross-sectional view of the infrared photodetector with doping superlattice structure according to a prior art. This device includes a substrate 1, a p+ emitter layer 2, doped superlattice layers 3, an N+ collector layer 4, an emitter electrode 5, and a collector electrode 6.
A configuration of the conventional photodetector made according to the conventional method is such that the quantum wells provided in the superlattice areas are arranged in a horizontal manner, therefore it is difficult for any incident light in a normal direction on the substrate to be absorbed in the superlattice areas.
To overcome such a problem, there has been used a substrate having an etched mesa structure suitable for absorbing more light incident in a normal direction, or a prism type of flat plate acting to change a propagating path of the incident light.
However, these configurations adopted to moderate the problem requires to use considerably expensive equipment, such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE), to be used for making the desired superlattice structure, which therefore increases in a manufacturing cost.
Further, since the structures made advantageously do not exhibit a satisfactory absorption feature of the light incident in a normal direction, the structures further necessitate another auxiliaries separately provided thereto for an effective absorption of the incident light, moreover not compatible with existing processes for fabricating the required integrated circuits, making it difficult to integrate the conventional structure together with other integrated circuitry.